Mediator (TRAP/ARC/PC2) is a large (22–28 subunit) protein complex that binds RNA polymerase II and controls transcription from class II genes. The evolutionarily conserved core of Mediator is found in all eukaryotes. It binds RNA polymerase II and is probably critical for basal transcription but it also mediates activation and repression of transcription. During evolution the complex has acquired additional species-specific subunits. These serve as an interface for regulatory factors and support specific signalling pathways. Recent mechanistic studies are consistent with the hypothesis that Mediator marks genes for binding by RNA polymerase II whereupon it subsequently activates the preinitiation complex. It is further likely that Mediator coordinates the recruitment of chromatin-modifying cofactor activities.